|
Year |
2005 |
|---|---|
|
Question Number |
275. |
|
Category |
Musculoskeletal Trauma |
Question
All of the following statements about the properties of intramedullary nails are true EXCEPT
1- plate and screw devices produce greater torsional stiffness than closed section nails in femoral shaft fractures.
2- closed section nails have greater torsional stiffness than slotted nails.
3- the radius of curvature of the femur is greater than that of intramedullary nails commonly used.
4- increasing wall thickness results in increased bending stiffness.
5- titanium has a modulus of elasticity closer to cortical bone than stainless steel.
Figures
Answer (click for answer)
Preferred Response: 3 – the radius of curvature of the femur is greater than that of intramedullary nails commonly used
Explanation:
In order to answer this question correctly you have to understand that radius of curvature is a measurement of straightness (for lack of a better word). Something with a lower radius of curvature is more curved while something with a greater radius of curvature is more straight (draw circles with different radii and see for yourself). The article cited examined 800 femur specimens and determined their average radius of curvature to be around 120cm (interestingly, left femurs were straighter than right femurs in 67% of bilateral specimens – the authors had no reason why). They also examined 8 common IM femoral nails and found them to have a radius of curvature of between 186 and 300 cm (or straighter than the femurs). The authors believe that this difference is the reason that many nails can cause anterior cortical penetration and has caused device companies to start to decrease the radius of curvature of their nails. Interestingly, long stemmed total hip systems have radii of curvature between 127-200 cm. If you didn’t know all of that, you might have been able to eliminate the other answers as follows:
Intramedullary nails provide relative stability to the fracture. These leads to a small amount of motion at the fracture site, callous formation and remodeling with healing. Plate and screw fixation imparts absolute stability or rigid fixation (at least when done properly) so you can eliminate answer 1. Closed section nails have been shown to have much higher torsional stiffness than open section (or slotted) nails, although they have the same bending stiffness when implanted. Eliminate answer 2. Increasing wall thickness does increase bending stiffness proportionally (you try to get the largest diameter nail you can without excessive reaming for this reason). Eliminate answer 4. We should all know answer 5 is a correct statement. The chart of the modulus of elasticity of common orthopaedic implants is in multiple textbooks and titanium has the closest to cortical bone, while stainless steel is much stiffer.
Reference: RECOMMENDED READINGS: Simon SR (ed): Orthopaedic Basic Science. Rosemont, IL, American Academy of Orthopaedic Surgeons, 1994, pp 277-323.
Tencer AF, Johnson KD: Lower extremity fixation, in Tencer AF, Johnson KD (eds): Orthopaedic Trauma: Bone Fracture and Fixation. Philadelphia, PA, Lippincott Williams & Wilkins, 1994, pp 249-304.
Egol KA, Chang EY, Cvitkovic J, et al: Mismatch of current intramedullary nails with the anterior bow of the femur. J Orthop Trauma 2004;18:410-415.